This invention relates to casting of continuous length foamed bunstock from a thermosetting fluid mix, more especially polymeric isocyanates such as polyurethane foams, but is likewise applicable to other expandable, thermosetting foam mixes. The invention relates in particular to improvements in apparatus for and method of manufacturing such foam product of improved density gradient and cell isotropicity.
Foamed plastic bunstock is used extensively in the manufacture of bedding (mattresses, pillows), furniture and automotive upholstery, thermal and sound insulation, and the like. Selfgassing polyurethane mixes are currently used predominantly. For many of these applications it is not practical to mold or cast the polyurethane foam directly in its desired final shape or form, especially, where physical characteristics of maximum uniformity of density, resiliency and the like are important. Usually it is more economical, and sometimes it is unavoidably necessary from a practical standpoint, to produce the product in large cast buns or billets of standard modular dimension, and then cut these into sections of desired shape. Bun molding has been commercially practiced for some time, both on a batch or individual bun-forming basis, and more recently in a continuous process wherein the foam mix is deposited in a moving mold to produce a bun of uninterrupted length. This is then sawed, sliced, etc., into appropriate lengths for ultimate product fabrication, as well as interim convenience of handling, shipping and storing.
In order to avoid waste in converting the ascast product to its final shape, one of the important considerations is to obtain a bun that has a flat top or minimum cresting or "bread-loaf" configuration; in other words, is of as nearly rectangular cross section as possible. This "bread-loafing" effect is a characteristic result of methods heretofore used in producing buns. Obviously if there is a substantial crest or hump in the top surface of the bun as produced, there will be a significant scrap loss upon skiving or cutting the billet into slabs to get flat, parallel surfaces. Since the foamed plastic generally has a relatively high unit value and since commercial production of the foam runs in the millions of pounds annually, any substantial scrap loss aggravates the cost of the finished product.
If there are splits, voids, bubbles and other discontinuities in the body of the foam stock, which has also been a common difficulty, portions of the foam block containing these must be cut out, thus producing further scrap loss. Additionally, nonisotropic cellular formation in the stock impairs the physical properties of the finished product, so a high degree of uniformity in the shape and size and axis orientation of the foam cells produced is accordingly desirable.
Although it might appear to be an easy solution, for ensuring a dimensionally uniform product, simply to confine the developing foam by suitable fixed molding or shaping means, this is not in fact readily accomplished, especially in practical commercial practice involving production of as much as several million pounds of product monthly in a single plant. During the development of the foam several actions and reactions take place simultaneously, and in some cases competitively. That is, there is generation of a gas to produce the foamed cellular properties, and at the same time there is a polymerization reaction taking place, leading to gellation or rigidification of the walls of the cells to impart the desired degree of resiliency and body in the product.
Prior attempts to produce foamed bunstock in a continuous manner generally involve depositing a reactive foam mix on a lower conveyor surface, such as a continuous paper web drawn over a stationary bed or pour board. Side restraints are also used to complement the lower surface, forming a U-shaped mold, looked at in cross section, of extended length. See for example U.S. Pat. No. 3,152,361. In order to get a finished foam bun of uniformly rectangular cross section, smoothing or "ironing" belts, aprons, rollers and the like have been applied to the surface of the advancing foam during its development. Examples of this are seen in U.S. Pat. Nos. 3,123,856, 3,553,300 and 3,655,311. This results in a tendency towards densification or compacting of the product, particularly at its upper surface, which is undesirable and this non-specification portion must usually be cut off and discarded.
To get around this, other attempts have been made to produce continuous flat top bunstock without any vertical confinement, using synchronously moving side conveyors complementing the lower conveyor surface and exerting a lateral effect on the developing foam bun. Some of these systems are quite complicated mechanically and troublesome to adjust and maintain. See for example U.S. Pat. Nos. 3,091,811 and 3,719,734; also British patent Nos. 1,225,968 and 1,235,915.
Potentially, what appeared for a while to be one of the more promising developments in continuous foamed bunstock production was a so-called upside-down process in which the lower conveyor element forming the bottom of the moving U-shaped mold is trained to move downward, relative to horizontal side elements, during that portion of the run where the maximum expansion occurs. The desired objective was to keep the upper surface of the developing foam at a substantially constant, horizontal level, while allowing the body of the foam to expand downwardly to meet the declining portion of the conveyor run. Typical examples of this approach are seen in U.S. Pat. Nos. 3,325,823 and 3,560,599. It is postulated that by causing the bottom of conveyor to descend relative to fixed side wall elements, there is a reduction of the frictional restraint at the interface of the foam stock and side conveyor elements, and that this would reduce the tendancy to form a relatively large radius in the bunstock at the intersection of the side and top surfaces. Unfortunately this prior teaching has not afforded a practical solution on account of both technological and economic problems with the proposals advanced. As a result, the industry has turned to those mechanically more complex systems of attempting to provide side lifting arrangements for the developing foam, as for instance those shown in U.S. Pat. No. 3,719,734 and the British patents mentioned above. Not only is there substantial equipment expense and added maintenance involved, there must also be a willingness to compromise on the quality of product obtained. For example, so-called "tin splits" can become critical, forcing an operator to resort to mix compositions that are not as favorable from the standpoint of foam resiliency, sag resistance, thermal properties or uniform density. There is the further disadvantage in that a given installation does not have much latitude or flexibility to permit use of different foam mix compositions, rates of production or ambient operating conditions from those for which the equipment was specifically designed.
Another area of difficulty encountered in prior systems has been that of preventing washback or undercutting of the developing foam on the pour board. This problem arises in part by uneven distribution in the advancing foam mix of portions of substantially different "age"; that is, portions of the liquid mix deposited on the conveyor at materially different times. The usual manner of depositing the mix on the moving conveyor is to transverse a mixing head back and forth across the width of the conveyor, laying down a zig-zag path of the mix on the advancing conveyor web. If the mixing head traverse rate is not properly coordinated with the throughput rate of liquid mix, the conveyor speed and the pitch of the pour board beneath the mixing head, substantial deviation may arise in average age of the liquid mix in any transverse section taken along the pour board downstream of the mixing head. Additional problems arise from portions of the mix, which have begun to foam, floating backwards against the direction of conveyor travel, while unreacted (non-foaming) portions of the liquid are carried forward and mixed with other portions downstream in which the foaming reaction has already progressed much further.